Project Abstract Transcranial electrical stimulation (tES) methods, principally transcranial direct current simulation (tDCS) and transcranial alternating current stimulation (tACS) are neuromodulation techniques that have been the subject of great recent interest. In typical tDCS procedures a pair of large electrodes (e.g., 25cm2) is attached to the scalp and a constant current of 1-2 mA passed between them for periods of 10-30 min. In tACS, the constant current intensity is similar, but an alternating sinusoidal waveform is usually employed. tES strategies have been indicated for a wide range of conditions, including stroke rehabilitation, treatment of epilepsy and for improving cognitive, motor, language and memory performance in healthy subjects. Details of underlying mechanisms of both tDCS and tACS remain unclear. It has been assumed that tDCS effects are greatest in brain structures nearest stimulating electrodes and that these structures experience the largest electric fields or current flow. At low intensities (ca. 1 mA) tDCS excitatory effects have been observed in structures under more positive electrodes and inhibitory effects under negatively polarized electrodes. It has been hypothesized that this is because externally applied field either depolarizes or hyperpolarizes resting membrane voltages in targeted tissue, leading to increased excitability or inhibition respectively. However, there is also evidence that at 2 mA intensity, increased excitability is observed regardless of polarity. Apart from factors relating to the subject initial state, neuroanatomy and CSF volume, it has also been suggested that major contributions to variability between individual sessions of a study or may be inconsistencies in electrode application protocols. Knowledge of the exact distribution formed within the brain by externally applied currents may clarify many study outcomes. The effects of different current application protocols, electrode designs and even prior study procedures could easily be distinguished. Most importantly, correlation of internal current density and electric field distributions with brain activity measures will help researchers attempt better detailed explorations of mechanism. In these studies we propose to investigate dosimetry, replication, and correlations between current density distributions and brain activity levels found in a previously validated task. Images of electrical field and current density distributions will be related to functional measures (fMRI) and behavioral measures of tDCS effect. The outcome of these analyses will be the first steps towards customized current delivery, direct insight into the impact of neuroanatomic variation on tDCS delivery and the first precise investigations into existing hypotheses of tES mechanisms.